Accumulators
Usage
1. Accumulation of energy: Achieving a large flow rate and compensating from internal leakage
2. Absorption of pulsations: Reducing noises and pulsations
3. Absorption of shocks: Used as a shock absorber Type
1. Bladder type: Separates gas from oil by a rubber bladder.
2. Diaphragm type: Sometimes used as a small
accumulator.
3. Piston type: Shaped in the form of a cylinder without a rod.
4. Spring type: Often used to prevent pulsations.
5. Weight loaded type: Designed for large-scale machinery and consisting of a cylinder and a plummet.
Figure 11.10 shows the accumulated condition of a bladder-type accumulator. V3 – V2 is equal to the output volume.
Cautions on Usage of Accumulators
(1) Accumulators should be vertically positioned so that their oil ports face down.
(2) Pre-charge pressure should be approximately 85 to 90 percent of the minimum working pressure, and should not be less than 25 percent of the maximum working pressure.
(3) Inert nitrogen gas (N2) should be used, while oxygen is strictly prohibited.
Changing Conditions of Gas
Changing conditions of gas under accumulation can be classified into isothermal, adiabatic, and polytropic changes.
(1) Isothermal change
When accumulation and output flow take a long time, heat generated in the action is completely absorbed into the surrounding atmosphere. As a result, there is no noticeable temperature change. Such volumetric change in accumulation, in association with temperature, is called isothermal change. In this case, the polytropic index is one.
(2) Adiabatic change
When accumulated gas is flashed out instantly, the gas expands or contracts with no heat exchange with the surrounding atmosphere. This type of volumetric change is called adiabatic change. Its polytropic index is 1.4.
(3) Polytropic change
No heat transfer takes place in adiabatic change, while all generated heat is transferred in isothermal change. In reality, pressure accumulation and discharge are accompanied with gas movement. The gas temperature, thus, changes after these phases with heat exchange. Temperature change caused by accumulation and discharge is not parallel with external temperature change. The gas temperature increases as the pressure accumulates, resulting in larger energy accumulated than in the case of isothermal change. The gas temperature, on the other hand, decreases as the pressure is released, generating a smaller amount of output flow. Polytropic change is generally obtained by the mean working pressure and the passage of time.
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